Get a subscription from a topic

In pubsub-usage.rst we have:

``` python
client = pubsub.Client()
topic = client.topic('topic-name')
sub = pubsub.Subscription('sub-name', topic)
```

I think we should consider changing this to

``` python
client = pubsub.Client()
topic = client.topic('topic-name')
sub = topic.subscription('sub-name')
```
